In prior art, numerous different automatic exposure control (AEC) systems are known, which have been applied in connection with many different imaging solutions. In diagnostic X-ray imaging, exposure automation plays a very important role because its incorrect operation may lead to over- or underexposure and, consequently, to an unnecessary increase in the radiation dose received by the patient and to uncertain or even false diagnoses due to poor image quality. Failed exposures can of course be corrected by re-imagings, but they further increase the radiation dose received by the patient and, on the other hand, also cause extra work.
The operation of the automatic exposure systems of the film-based apparatuses currently in use is typically based on empiric methods, in which a huge number of test exposures are done for new film/intensifying screen combinations using different imaging values of the imaging apparatus and varying the thickness, generally in range of 20-80 mm, of the acrylic plate typically used to represent the object to be imaged. Depending on the details of operation of the automatic exposure system used in each case, its various parameters are adjusted according to the results obtained until a sufficiently constant degree of film darkening is achieved in all circumstances. Each time when a new film/intensifying screen combination appears on the market or when more demanding tolerance requirements are set, such measurement series have to be carried out anew.
In the solutions according to the prior art in which electric imaging technology is applied, the automatic exposure control is typically realized by using a slightly different principle, because a dynamic range of an imaging detector is notably wider than that of a film and over-exposure of some areas of the object is not a problem in these solutions, because one will be able to get those areas visible anyway later by means of image processing. The principles of tuning of the automatic exposure system is nevertheless the same as in the film based technology, i.e. bundles of acrylic plates are exposed and such imaging parameters are looked for, empirically, by which the level of the signal being created at the detector remains substantially the same.
The commonly used tuning method as described above is thus based on imaging of a homogeneous plate, generally made of acrylic, simulating the object to be imaged. The periodic inspections carried out by the authorities to test the exposure automatics are based on the same method, which in respect of repeatability of the test is, evidently, necessary. However, in an actual imaging situation, the object being imaged is not necessarily homogeneous. For example, the breast tissue imaged in mammography is by no means homogeneous, nor is the breast of standard size or shape in its overall dimensions. While in mammography the object is typically also imaged from different projections, its shape and position in the imaging area may vary for this reason, too.
For the purpose of reducing problems of automatic exposure systems being based on a single detector signal only, approaches that make use of several AEC signals have been developed. In some of them, signals of those detectors have been arranged to be automatically disregarded in which the amount of radiation received exceeds some preset level. Such a limit level may be set such that, in order to reach it, it is obvious that the detector is located completely or partially outside the tissue being imaged. Solutions have also been developed, in which that one out of the few detectors is selected, whose signal appears to be the most suitable. While these solutions have significantly reduced exposure errors, they still don't necessarily guarantee that also the lightest regions of the image being formed in the imaging will become dark enough.
In particular in mammography, there are special problems of its own being caused by not only the abovementioned non-homogeneity of the breast tissue in regard to attenuation of the radiation used for the imaging as such, but also by the possibility of imaging, in addition to the breast tissue, a silicon implant or muscular tissue of the armpit area, for example. Further, in some imaging modes, even parts of the apparatus used for the imaging may get included in the image. When a cancer tumor often locates just within the densest places of the breast tissue, it would be essential to get especially those places imaged adequately black. One is not necessarily able to achieve this, though, in case the exposure control is based, for its essential part or completely even, on such a signal which corresponds a region which attenuates the radiation even more than the densest place of the breast tissue (in regard to the attenuation of radiation used for the imaging). Such an outcome may occur in the case if one decides to elect the lowest signal to be the abovementioned most suitable signal, for example, but this very signal in question happens to be measuring radiation having penetrated e.g. a silicone implant.